In the last few years, the number of commercial cellular communications systems (of which cellular telephony is but one example) has risen dramatically. Increased use of cellular systems among consumers has given rise to a demand for higher capacity systems. In the increasingly competitive enterprise that cellular telephony has become, higher capacity systems necessitate more efficient and more cost effective systems. One of the overall limiting factors in the implementation of a cellular system is the bandwidth allocated to the system. In general, an increase in available or usable bandwidth allows a system to service a larger number of users or to provide those users with higher quality service. In the last few years, the actual bandwidth allocated to cellular telephony has remained nearly constant. Consequently, the cost of obtaining band width has risen dramatically. Thus, efficient methods of utilizing existing bandwidth are essential to increasing the capacity of cellular systems and thereby maintaining commercial viability.
In cellular communications systems, the geographic area of service of the system on the surface of the earth is generally divided into an interlocking hexagonal grid. Each hexagonal grid element is called a cell. Cells may be geographically fixed or may be defined by spot beams from a satellite. Communications signals may be transmitted to and received from consumers within each cell through the use of orbiting satellites.
In a satellite-based cellular system, the overall RF frequency band of operation is divided into a number of unique, non-overlapping RF sub-bands. Multiple cells may use the same frequency sub-bands. When a single sub-band is used by cells that are close to one another, the system may experience mutual interference between the cells. Mutual interference between cells utilizing the same frequency sub-band is known as co-frequency interference. To reduce co-frequency interference, sub-bands are allocated in a frequency pattern such that cells having a common frequency sub-band are geographically located a predetermined minimum distance from one another. The frequency pattern by which the geographic cells are allocated may be referred to as the frequency reuse pattern. The number of sub-bands in the overall system and the accompanying frequency reuse pattern may be referred to as the frequency reuse scheme. For example, the frequency reuse scheme may segment the total system bandwidth into seven sub-bands in a 7-to-1 frequency reuse scheme.
Satellite-based cellular systems may reduce co-frequency interference by increasing the number of sub-bands at the expense of reducing the overall capacity of the system. However, it is not feasible in many instances to increase the number of subbands because the total bandwidth is limited and each sub-band must have a predetermined minimum bandwidth to support communication (in addition to the reduced capacity of the system, i.e., with a fixed number of cells, the more sub-bands there are, the less that frequency is reused throughout the entire system.) Thus, the constraint that each sub-band must have a minimum pre-defined bandwidth limits the number of sub-bands into which the total system bandwidth may be divided. The limitation on the number of sub-bands similarly limits both the available frequency reuse patterns and the distance between cells utilizing the same frequency sub-band.
Alternatively, the system may be designed around a specified level of interference between cells operating in the same frequency band. The level of co-frequency interference is generally proportional to the distance between cells using a common sub-band. Therefore, when the system sets the maximal co-frequency interference, the system must similarly set a minimal separation between cells operating in the same frequency range. Specifying the minimal separation between cells operating in the same frequency sub-band dictates the frequency reuse plan and the total number of sub-bands necessary to implement the frequency reuse plan because the size of the hexagonal cells of the cellular system is constant. Determining the total number of sub-bands also sets the bandwidth allocated to each sub-band because the total bandwidth of the system is limited.
The increasing commercial exploitation of cellular communications systems demands that increasing bandwidth be allocated to each cell to support increased user demand because the bandwidth allocated to each cell is proportional to the number of voice or data users that can be serviced simultaneously in each cell. In general, increasing the bandwidth per cell increases the number of customers that the system can service in the cell at any one time, thus increasing overall profitability. However, because the overall bandwidth of the cellular system is limited, increasing the bandwidth allocated to each sub-band must necessarily reduce the number of sub-bands into which the system can be segmented. A reduction in the number of sub-bands serves to alter the frequency reuse pattern to cause the distance between cells utilizing the same frequency sub-band to decrease which in turn increases the co-frequency interference between these cells. In order to implement a higher volume cellular system, more bandwidth must be allocated to each cell. This means fewer sub-bands and consequently greater co-frequency interference from cells utilizing the same frequency sub-band because the cells must be closer together.
A method of reducing the co-frequency interference may allow the system to allocate cells using the same sub-band at a lesser distance from one another. Reducing the distance between cells in turn may allow the system to function with fewer sub-bands at a correspondingly greater bandwidth per sub-band. Greater bandwidth per sub-band in turn allows more simultaneous users, and in turn increased profitability to the service provider.
Additionally, one of the main design constraints when designing a digital cellular communications system is the bit error rate (BER). The BER generally represents the rate at which errors occur in a digital data stream between a transmitter and receiver. In digital data streams, a bit error occurs when the transmitter transmits one digital value, but the receiver outputs a different digital value to the user. The bit error rate is dependent upon the quality of the connection between the transmitter and receiver. The quality of the connection between the transmitter and receiver is affected by the amount of noise or interference in the environment through which the digital data stream travels.
Satellite-based cellular systems may compensate for noise or interference by modifying the gain pattern of an antenna on the satellite which transmits or receives the digital data stream. On the receiver side, the antenna gain represents the sensitivity of the antenna to the incoming signal. On the transmission side, the antenna gain is proportional to the power level at which outgoing signals are transmitted during transmission. Cellular systems are generally designed to service users anywhere in a given cell, providing a predetermined maximum BER (termed required design BER). If at any point in the ground cell the system is not able to maintain the predetermined maximum BER, communications with the user at that point will be noticeably impacted. Thus, the antenna gain (and gain pattern) for an entire cell must be sufficient to support a minimum signal level regardless of the noise or interference experienced by of the user in the cell.
In satellite-based cellular systems the quality of the connection between transmitters and receivers may be expressed in terms of a ratio between the desired signal or carrier level and the noise plus interference level. The ratio between carrier strength and the noise plus interference level may be described as C/(N+I). In many instances, co-frequency interference may dominate the noise plus interference term, thereby allowing the above expression to be approximated as C/I. However, in many systems background noise is a significant factor and hence the ratio C/(N+I) is generally of interest. The ratio C/(N+I) is important in the design of a cellular system because it is indicative of system performance. A high C/(N+I) ratio yields an improved BER since the bit error rate (BER) is determined by the carrier to noise plus interference ratio C/(N+I). Consequently, a cellular system having a high C/(N+I) ratio will have a low BER and vice-versa.
In the past, cellular systems have been proposed which select a level of performance by choosing a maximum BER for users in the overall system. Thus, conventional systems impose a uniform level of performance across an entire geographic cell. In conventional satellite-based communications systems, the minimum performance of the system (and thus the largest acceptable BER) is based on the location in the cell having the lowest C/(N+I) ratio.
In other words, past satellite-based cellular systems provide a minimum antenna gain at the point in a cell having the lowest C/(N+I) ratio that is sufficient to ensure a BER less than the maximum predetermined BER. In past antenna designs, the minimum antenna gain at the cell point of lowest C/(N+I) ratio represented an antenna gain level far below the maximum antenna gain for the antenna. Generally, conventional antennas provide an antenna gain pattern across a cell, wherein the pattern has a peak gain level at the center of the cell and minimum gain levels at the boundaries of the cell. The minimum signal (or carrier) level at the cell boundary must exceed the noise plus interference level by a desired amount to achieve a predetermined BER. Thus, in past antennas, when the carrier level at the cell boundary is boosted to just exceed the noise plus interference level, the carrier level at the center of the cell greatly exceeds the noise plus interference level. Consequently, a large amount of power is wasted in providing service to users at the center of cells.
In the past, antennas for cellular satellite systems have been designed to afford minimum gain at the boundary of a geographic cell to assure a satisfactory signal level. Past antennas have also been designed to reduce sidelobes to a desired level to reduce interference. In a conventional cellular satellite system, the antenna gain pattern is known for an entire spot beam which thereby determines the C/(N+I) at any location throughout the system. However, in conventional antennas, the antenna gain pattern does not afford a constant C/(N+I) over the cell. Instead, the carrier to noise plus interference C/(N+I) ratio at the center of the cell is much larger than at the boundaries of the cell. When the antenna provides a C/(N+I) ratio that is not constant over the entire cell, resources such as power are wasted because achieving a higher C/(N+I) ratio than the minimum required ratio typically does not add any value to the system.
Conventional antennas may include phased arrays, reflector-based antennas and the like. An antenna gain pattern is determined by controlling the phase and amplitude of signals transmitted from, and received by, a phased array. In antennas based on reflectors, the antenna gain pattern is determined by the size and contour of the reflector and the antenna feed design.
An example of a reflector-based antenna is disclosed in U.S. Pat. No. 5,134,423 issued to Haupt ("Low Sidelobe Resistive Reflector Antenna"). The Haupt patent discloses an antenna with a shaped reflector having a parabolic dish antenna composed of a dielectric upon the surface of a shaped reflector. A metallic coating has been deposited on the surface of the shaped reflector. The thickness of the tapered metallic coating on the surface of the dish is varied according to the radius of the dish to achieve electromagnetic tapering. When the tapered metallic coating is applied, it provides low resistivity at greater thickness and progressively higher resistivity as less metal is deposited. The metallic surface may be composed, for example, of conductors such as aluminum, copper, steel, iron, gold, and silver and may be deposited onto the surface of the dish through deposition techniques such as sputtering, evaporation, electrodeposition, and spray painting.
A need remains for an improved antenna gain control method and apparatus. It is an object of the present invention to meet this need.